a) Press “Windows Logo” + “R” keys on the keyboard to open the “Run” command box.
b) Type “regedit” in the “Run” command box and press “ENTER”.
c) Now, navigate to following key:
d) In right-side pane, create new DWORD “NoReadingPane” and set its value to 1.
(To create a new DWORD, click on the option “Edit” at the top, select “New” and then select the option “DWORD” from that sub-menu.)
e) Restart or log off your system and "Preview Pane" button will get removed from Windows Explorer command bar.
To restore the "Preview Pane" button, you can either remove DWORD or change its value to 0.
(Taken from: https://answers.microsoft.com/en-us/windows/forum/windows_7-desktop/how-to-hide-the-file-preview-pane-on-the-browse/36e8da9c-10c0-4ac0-ab16-284fea930075 and placed here to make it easier to find.)